How Movements Drive Population Development of Harbour Seals and Grey Seals in the North Sea
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SEALS IN MOTION How movements drive population development of harbour seals and grey seals in the North Sea SOPHIE MARIE JACQUELINE MICHELLE BRASSEUR SEALS IN MOTION How movements drive population development THESIS COMMITTEE of harbour seals and grey seals in the North Sea PROMOTOR Prof. Dr. P.J.H. Reijnders, Professor of Ecology and Management of Marine Mammals, Wageningen University and Research. SOPHIE MARIE JACQUELINE MICHELLE BRASSEUR CO-PROMOTOR Dr. G.M. Aarts, Researcher Marine Ecology, Wageningen University & Research. Thesis submitted in fulfilment of the requirements for the degree of doctor OtHER MEMBERS at Wageningen University Prof. Dr. A. D. Rijnsdorp, Wageningen University & Research. by the authority of the Rector Magnificus, Prof. Dr. T. Härkönen, Swedish Museum of Natural History, Stockholm, Sweden. Prof. Dr A.P.J. Mol, Dr. D. Thompson, University of St Andrews, Scotland. in the presence of the Prof.dr. P.J. Palsbøll, University of Groningen, the Netherlands. Thesis Committee appointed by the Academic Board to be defended in public This research was conducted under the auspices of the Netherlands Research on Wednesday 30 August 2017 School for the Socio-Economic and Natural Sciences of the Environment (SENSE). at 1:30 p.m. in the Aula. 2 3 hoofdstuk Sophie Marie Jacqueline Michelle Brasseur Seals in Motion - How movements drive population development of harbour seals and grey seals in the North Sea 176 pages. PhD thesis, Wageningen University, Wageningen, the Netherlands (2017). With references, with summaries in English and Dutch. ISBN: 978-94-6343-612-0 DOI: https://doi.org/10.18174/418009 FREQUENTLY, GREY SEALS (FRONT) AND HARBOUR SEALS (BACK) SHARE THE SAME HAUL OUT 4 (PHOTO: HANS VERDAAT) 5 Seals in motion hoofdstuk ALL SEAL TRANSMITTERS WHERE DEPLOYED WITH THE HELP OF The “WADDEN UNIT” oF THE 6 MINISTRY OF ECONOMIC AFFAIRS (PHOTO: J. BRASSEUR) 7 Seals in motion hoofdstuk TABLE OF CONTENTS 1. Introduction and outline of thesis ........................................................................ 12 2. Echoes from the past: regional variations in recovery within a harbour seal population ......................................................................................... 20 3. Earlier pupping in harbour seals, Phoca vitulina ................................................ 44 4. Directional breeding migration of harbour seals in the Wadden Sea .............. 54 5. Rapid recovery of Dutch grey seal colonies fuelled by immigration ............... 76 6. Resident and transient grey seals in the North Sea ............................................ 98 7. Synthesis ................................................................................................................. 112 Addenda: Summary ................................................................................................................ 122 Nederlandse samenvatting ................................................................................... 126 References .............................................................................................................. 132 Contributing authors ............................................................................................. 150 Publications ........................................................................................................... 152 Acknowledgements ............................................................................................... 166 FROM THE AIR, THE DIFFERENCE BETWEEN THE TWO SPECIES IS NOT ALWAYS ObvIOUS 8 (PHOTO: SOPHIE BRASSEUR) 9 Seals in motion hoofdstuk WHEN EQUIPPED WITH A TRANSMITTER, SEALS ARE ALSO WEIGHED AND MEASURED. SEVERAL TEAMS WORK IN PARALLEL TO LIMIT THE TIME OF CAPTIVITY TO AROUND ONE HOUR. 10 (PHOTO: STEVE GEELHOED) 11 Seals in motion 1. Introduction and outline of the thesis PHENOLOGY, MIRRORED ANNUAL CYCLES The harbour seal is the smaller seal, with females in the European sub-species at- taining up to 150 cm and males 160 cm: they are amongst the smallest harbour seals 1. INTRODUCTION (McLaren 1993, Härkönen & Heide-Jørgensen 1990). Females become sexually ma- ture at 3 to 4 years of age, males mature a year later (Härkönen & Heide-Jørgensen 1990), though they might not be able to participate in breeding for several more years due to competition from older animals. After the first parturition, females AND OUTLINE OF annually give birth to a single pup, weighing approximately 9 kg. In very rare oc- casions, twins may occur (Spotte 1982, Olson et al. 2016). In the Netherlands, peak in pupping occurs in early June (Reijnders et al. 2010b). Pups have usually moulted into an adult fur before birth, though a small percentage is born with lanugo. As pups can swim within hours after their birth, breeding sites may include both tidal THE THESIS flats and areas that remain dry irrespective of the tides. Lactation duration may vary between 15 and 28 days, with a median before 21 days (Cordes & Thompson 2013, Thompson & Wheeler 2008, Wieren 1981). During lactation, the mother and pup The Netherlands is one of the most densely populated countries of the world, with pair may alternate between being on land and in water. Suckling is often observed approximately 25% of now inhabited land reclaimed from the sea. Despite this, in on land but may also be aquatic (Burns et al. 2009). In some cases, mothers may the waters bordering this country wild populations of two large mammal species leave their pup to feed for several hours. Oestrus occurs postpartum after lactation have been able to recover from complete extinction in the case of the grey seals (Reijnders 1990, Pomeroy 2011). Different to most seal species, harbour seals are (Halichoerus grypus ), or near extinction in the case of the harbour seals (Phoca known to breed in leks at sea (Hayes et al. 2006, Boness et al. 2006, Hayes et al. vitulina) in the course past century. In this thesis, I describe the recoveries through 2004, Parijs Van et al. 1997). Pups are weaned abruptly after which they go through analysis of long-term monitoring of both species and tracking data collected in the a period of post weaning fast, during which they lose between 2 and 5 kg (Prewitt framework of recent environmental impact assessment studies. et al. 2010, Muelbert et al. 2003). Newly weaned pups may scatter approximately 50 The harbour seal and the grey seal are currently conspicuous members of the ma- d after birth to haul-outs near feeding sites away or close to the birth sites (Small et rine mammal fauna in the North Sea. The harbour seal has a circumpolar distribu- al. 2005, Bjørge et al. 2002, Härkönen & Harding 2001, Blanchet et al. 2014). After tion on the Northern Hemisphere with four sub-species distributed on either side of breeding, harbour seals may have a brief period of foraging before their annual the great oceans: the eastern North Atlantic P. vitulina vitulina, the western North moult. Moult in Dutch waters occurs one or two months after the breeding season Atlantic P. vitulina concolor, the eastern North Pacific P. vitulina richardii, and the with a peak in August: adult animals moult latest (Cronin et al. 2014, Härkönen et western North Pacific P. vitulina stejnegeri. The grey seal is only found in the North al. 1999). Based on the loss of tracking devices glued to their fur (this thesis), bree- Atlantic, and three distinct populations are recognized: the Northeast Atlantic the ding females would seem to moult last. Between the moulting period and the next Northwest Atlantic, and the Baltic Sea grey seal (Reijnders et al. 1993, Hall et al. breeding period, harbour seals have what might be called a foraging period. There 2009, Burns et al. 2009). The nomenclature has been subject to some recent debate are indications that there is some periodicity in the feeding intensity (Brasseur & and the most recent suggestion is to name the Atlantic grey seal Halichoerus grypus Fedak 2002). The harbour seals show intensified feeding until early spring and, as atlantica while changing the name of the Baltic sub-species to Halichoerus grypus water temperature rises and the seals presumably have reached an optimum weight, grypus instead of H. grypus macrorhynchus or H. grypus Balticus (Olsen et al. 2016). less intensive feeding until the breeding season (Renouf & Noseworthy 1990). This thesis concentrates on the subspecies of harbour and grey seals occurring in the Netherlands, thus the North Sea, only using the species name. FIGURE 1. COMPARISON OF ANNUAL CYCLES FOR SEALS IN DUTCH WATERS Grey seals are the largest phocids in the temperate North Atlantic. Compared to other phocids, grey seals exhibit a high level of sexual dimorphism; the Eastern Atlantic grey seal males attain a length of 210 cm in average while females are sig- 12 13 Seals in motion 1. Introduction and outline of the thesis nificantly smaller and reach in average to 185 cm (McLaren 1993). Adult males can be up to 300 kg, adult females up to 200 kg. A function of the larger size attained by males is thought to be to dominate reproductive opportunities within breeding colonies (Anderson & Fedak 1987). Male grey seals are sexually mature at 5-6 years, though they may not be socially mature until the age of 8 or older. Females are sexually mature at 3-5 years of age and then pup annually. In the Netherlands, peak in pupping occurs in December (Figure 1). The time of year when grey seals give birth varies between regions, and there is a clockwise cline in the mean birth date around the UK from August and September